High temperature alloy

ABSTRACT

A carbide-strengthened castable cobalt-base superalloy possessed of excellent high-temperature properties and improved ductility and containing controlled amounts of chromium, nickel or iron, molybdenum, tungsten, tantalum, titanium, zirconium, carbon and the balance cobalt, along with conventional residual alloying elements and incidental impurities.

United States Patent Morrow, III et al.

Oct. 1, 1974 HIGH TEMPERATURE ALLOY Inventors: Hugh Morrow, III; WilbertP.

Danesi; David L. Sponseller, all of Ann Arbor, Mich.

Assignee: American Metal Climax, Inc., New

York, NY.

Filed: July 16, 1973 Appl. No.: 379,814

US. Cl. 75/171, 148/32 Int. Cl. C22c 19/02 Field of Search 75/171, 170;148/32, 32.5

References Cited UNITED STATES PATENTS 3/1969 Wheaton 75/171 PrimaryExaminerRichard 0. Dean Attorney, Agent, or FirmHarness, Dickey & Pierce5 7 ABSTRACT 6 Claims, No Drawings HIGH TEMPERATURE ALLOY BACKGROUND OFTHE INVENTION The high stresses and elevated temperatures to whichcomponents of high-performance gas turbines are subjected has prompted acontinuing search for materials which provide for an optimum balance ofmechanical properties over the normal operating temperature range ofsuch engines. Among the various materials which have been developed forthis purpose, so-called superalloys of a nickel or cobalt-base have beenfound particularly satisfactory. Of the foregoing, cobalt-basesuperalloys are in widespread use because of their ability to be cast,producing components which are possessed of excellent mechanicalproperties, including high stress-rupture and tensile strengths atelevated temperatures and excellent high-temperature oxidation andcorrosion resistance. characteristically, most of the cobalt-basesuperalloys in commercial use are strengthened by a combination of solidsolution and carbide dispersion strengthening and employ comparativelyhigh quantities of chromium and refractory metals, of which tungsten isperhaps the most popular, although tantalum has also received someacceptance.

Typical of the several castable cobalt-base superalloys is the alloydisclosed in US. Pat. No. 3,432,294 comprising a carbide-hardenedalloynominally containing about 0.6 percent carbon, about 18 percent toabout 24 percent chromium, about 7 percent to about 15 percent nickel,about 6 percent to about 9 percent tungsten, about 2 percent to about 5percent tantalum, about 0.1 percent to about 0.5 percent titanium, about0.1 percent to about 1 percent zirconium with the balance beingessentially cobalt. While alloys of the foregoing type have been foundsatisfactory for the fabrication of various components forhigh-performance gas turbine engines, the high cost of such alloys hassomewhat detracted from a more widespread use thereof. In addition,continuing improvements in gas turbine engine designs, providing forstill greater efficiency and performance, has resulted in a continuingneed for superalloys of improved high-temperature mechanical propertieswhich can be produced at more reasonable costs. The cobaltbasesuperalloy of the present invention, as a result of the careful controlin type and quantity of the individual alloying constituents, providesfor a still further improvement in certain moderate and high-temperaturemechanical properties of such alloys, while at the same time providingcost advantages over similar type alloys heretofore known.

SUMMARY OF THE INVENTION The benefits and advantages of the presentinvention are achieved by a cobalt-base superalloy containing as itsessential alloying ingredients from about 18 percent to about 26 percentchromium, from about 7 percent to about percent nickel or, in thealternative, from about 7 percent to about 12 percent iron; from about2.5 percent to about 4.5 percent tungsten, from about 1 percent to about4 percent molybdenum, from about 2 percent to about 5 percent tantalum,from about 0.1 percent to about 0.4 percent titanium, from about 0.1percent to about 1.0 percent zirconium, from about 0.4 percent to about0.7 percent carbon, and with the balance consisting essentially ofcobalt, together with residual elements including manganese, silicon,boron, niobium and the like, present in conventional amounts normallyencountered as well as conventional incidental impurities. In accordancewith a preferred embodiment of the present invention, the nickelconstituent is entirely replaced by iron without a significant sacrificein the mechanical properties of the resultant alloy, but with asignificant cost saving attributable not only to the elimination of anappreciable quantity of nickel, but also enabling the use offerro-chromium and ferromolybdenum as a source of the chromium,molybdenum and iron alloying constituents.

Additional benefits and advantages of the present invention will becomeapparent upon a reading of the description of the preferred embodimentsand the specific examples provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The composition of thecobalt-base alloy comprising the present invention as herein describedand as set forth in the subjoined claims is defined in terms ofpercentages by weight unless clearly indicated to the contrary.

The usable proportions and the preferred proportions of the individualalloying constituents comprising the nickel-containing cobalt-base alloyin accordance with one embodiment of the present invention is set forthin Table 1.

TABLE 1 Nickel-Type Cobalt-Base Alloy Composition as low as possibleSimilarly, the usable proportions as well as the preferred proportionsof the individual alloying elements comprising the so-calledferrous-type cobalt-base alloy in accordance with a second embodiment ofthe present invention is set forth in Table 2.

TABLE 2 Ferrous-Type Cobalt-Base Alloy Composition Usable RangePreferred Range Ingredient (percent) (percent) Chromium 18 26 23 25Nickel up to 2 up to 1 Iron 7 12 9 l 1 Molybdenum 1 4 1 3 Tungsten 2.54.5 3.2 3.8 Tantalum 2 5 3 4 Titanium 0.1 0.4 0.2 0 3 Zirconium 0.1 1.00.3 0.6

TABLE 2-Continued Fcrrous'l' \pc Cobalt-Base Allo Composition as low aspossible The principal distinctions between the so-called nickel-typeand ferrous-type cobalt-base alloys as set forth in Tables 1 and 2,respectively, resides in the quantity of iron and nickel present in thealloys. The nickel-type alloy contains residual quantities of iron up toabout 2 percent in combination with from about 7 percent to about 12percent nickel, while the ferrous-type alloy contains nickel as aconventional residual in amounts up to about 2 percent, and with ironsubstituted therefor in an amount ranging from about 7 percent to about12 percent. In addition to the foregoing differences, the nickel-typealloy has permissible manganese and silicon contents of up to about 0.5percent, whereas the irontype cobalt-base alloy may contain manganeseand silicon in amounts as high as about 1 percent.

A principal distinction between the alloys defined in Tables 1 and 2from prior art type cobalt-base alloys is the presence of molybdenum inamounts of from about 1 percent to about 4 percent and preferably, fromabout 1 percent to about 3 percent in combination with lesser quantitiesof tungsten providing therewith unexpected improvements in certainhigh-temperature mechanical properties of the alloys, as well as anappreciable reduction in their costs. In both instances, the costsavings achieved by a substantial reduction in the tungsten content andthe replacement thereof with lowercost molybdenum has been achievedwithout substantially modifying the stress-rupture, tensile strength,

thermal expansion and solidification characteristics of the alloys. Atthe same time these modifications provide for an improved stress-ruptureand tensile ductility and a lower room temperature density of the alloy.The inclusion of molybdenum, therefore, as a partial replacement for thetungsten constituent provides for definite unexpected benefits in theproperties of the cobalt-base alloys accompanied by a cost savings whichis particularly marked in connection with the ferroustype cobalt-basealloy of Table 2, permitting the use of lower-cost ferro-chromium andferro-molybdenum as the source of the chromium, molybdenum and ironalloying constituents.

As set forth in Tables 1 and 2, the essential alloying constituents ofthe improved cobalt-base superalloys of the present invention comprisechromium, nickel or iron, molybdenum, tungsten, tantalum, titanium,zirconium, and carbon with the balance essentially cobalt and with theremaining constituents being present as residuals and incidentalimpurities which can generally be tolerated in the amountsas specified,at which level they do not adversely affect the high-temperaturephysical and chemical properties of the alloys. The manganese andsilicon constituents, for example, comprise conventional residuals inferro-chromium, which preferably is employed for preparing the alloy asset forth in Table 2 because of economic considerations. It is for thisreason that the manganese and silicon contents of the ferrous'typecobalt alloy are higher than that of the nickel-type cobalt alloy. Thesilicon constituent is present as a conventional residual in nominalamounts of about /2 percent in both the ferro-chromium andferromolybdenum materials of the types normally employed for preparingthe ferro-type cobalt alloy. A typical lowcarbon ferro-chromium alloyingmaterial nominally contains about 30.8 percent iron, 0.45 percentsilicon, 0.64 percent manganese and about 0.04 percent carbon, with thebalance consisting essentially of chromium. A typical ferro-molybdenumalloying material nominally contains about 38.6 percent iron, 0.62percent silicon, 0.04 percent carbon, and the balance comprisedessentially of molybdenum. As previously mentioned, the high permissibleiron content of the ferroustype cobalt-base alloy set forth in Table 2permits the use of the two lower-cost ferro-chromium and ferromolybdenummaterials, providing for substantial further cost savings in preparingthe alloys and accordingly, constitutes a preferred practice.

The cobalt and chromium alloying constituents impart the requisitehigh-temperature strength and corrosion and oxidation resistance to thecobalt-base alloy. Chromium contents above the upper limits specified inTables 1 and 2 are generally undesirable due to the precipitation ofphases resulting in brittleness of the resultant alloy, whereas amountsbelow the lower limits usually result in inadequate oxidation resistanceand a sacrifice in the physical strength properties of the alloy.

The mechanisms by which the cobalt-base superalloy is strengthenedinclude carbide strengthening and solid-solution strengthening. Theformer effect derives mainly from the MC carbide-forming constituentstantalum, titanium and zirconium. These elements form carbides that arelocated in the interdendritic regions or are dispersed in the matrix.The latter effect consists of solid-solution strengthening of the matrixmainly by tungsten, molybdenum and chromium. The stability of theresultant microstructure provides for superior durability of the alloywhen subjected to stresses at elevated temperatures as high as 2,000Fand above, which are characteristic of peak temperatures attained in thehot section of modern, high-performance gas turbine engines. Therefractory-type carbide-forming metallic elements are interrelated intype and quantity within the limits as set forth in Tables 1 and 2 toprovide for optimum physical and chemical properties of the resultantcobalt-base alloy. In this connection, the carbon content of the alloyis carefully controlled within the limits specified to produce therequired quantity of carbides.

In addition to the foregoing important alloying elements, the remainingalloying constituents enumerated comprise residuals whose presence isnot essential, but which can be tolerated up to the amounts specifiedwithout adversely affecting the properties of the alloy. For example,the niobium constituent comprises a residual usually associated withtantalum and while permissible in amounts up to 2 percent, preferably iscontrolled to levels below about 1 percent due to the improved oxidationresistance of the resultant composition. Other conventional residualsmay also be present in usual minimal quantities along with incidentalconventional impurities of the types which do not adversely affect themoderate and high-temperature physical and chemical properties of thealloy.

In order to further illustrate the alloy compositions comprising thepresent invention, the following examples are provided. it will beunderstood that the examples are provided for illustrative purposes andare not intended to be limiting of the scope of the invention as hereindescribed and as set forth in the subjoined claims.

EXAMPLE 1 A cobalt-base superalloy of the nickel type in accordance withTable l was prepared employing vacuum melting techniques to provide anominal composition of 24 percent chromium, percent nickel, 1.9 percentmolybdenum, 3.5 percent tungsten, 3.5 percent tantalum, 0.2 percenttitanium, 0.5 percent zirconium, 0.6 percent carbon, and with thebalance cobalt along with conventional residuals and incidentalimpurities. The resultant vacuum melted composition was investment castinto test specimens and subjected to tests at elevated temperatures.

EXAMPLE 2 A cobalt-base superalloy of the so-called ferrous-type inaccordance with the composition as set forth in Table 2 was preparedemploying vacuum melting techniques to provide a nominal composition of24 percent chromium, 10 percent iron, percent molybdenum, 3.5 percenttungsten, 3.5 percent tantalum, 0.2 percent titanium, 0.5 percentzirconium, 0.6 percent carbon and the balance cobalt and conventionalresiduals and incidental impurities. The vacuum melted alloy wasinvestment cast into test specimens and subjected to physical tests atelevated temperatures as in the case of the test specimens of Example 1.

Room and elevated temperature tensile properties and stress ruptureproperties of the alloys prepared in accordance with Examples 1 and 2were determined and the data are set forth in Table 3.

TABLE 3 Alloy- Alloy- Property Example 1 Example 2 Ultimate TensileStrength,

psi 74F [09,300 112,700 1400F 85,000 79,100 1800F 35,000 30,000 2000F19.100 18.300

0.2% Yield Strength, psi

74F 77,200 78,200 1400F 44,800 41,800 1800F 29,500 22,500 2000F 15.30015.800

71 Elongation 74F 3.0 2.0 1400F 15.2 20.0 1800F 26.8 37.6 2000F 21.625.2

71 Reduction Area 74F 2.9 3.0 1400F 12.4 16.6 1800F 25.7 39.1 2000F 21.629.9

IOU-Hour Rupture Strength,

psi 1600F 27,500 25,000 1800F 15,500 13.600 2000F 7,000 6,400

TABLE 3-Continued Alloy- Alloy- Property Example 1 Example 2 100-HourRupture Elongation. 7(

1600F 9.0 24.3 1800F 12.0 20.0 2000F 11.0 13.3

As will be noted, the alloy of Example 1 has superior high-temperaturetensile strength properties in comparison to the alloy of Example 2 dueto the presence of nickel in lieu of iron. Both alloys possessedexcellent rupture strength and excellent intermediate temperature,tensile and stress rupture ductility, the latter properties beinggenerally superior to those of similar cobalt-base alloys of the typesheretofore known. In addition to the foregoing properties, thecoefficient of thermal expansion of the two alloys were substantiallythe same as that of prior art cobalt-base alloys. For example, the alloyof Example 1 has an average thermal expansion coefficient (10"inches/inch/C) of 14.28 from room temperature to 500C and 16.18 fromroom temperature to 1,000C; while the alloy of Example 2 has averagecoefficients of 14.58 and 16.26, respectively, for the same twotemperature ranges. Similarly,

- the alloy of Example 1 has a density at room temperature of 8.53 g/cc(0.308 lbs./in while the alloy of Example 2 has a density of 8.58 g/cc(0.309 lbs/in).

While it will be apparent that the invention herein disclosed is wellcalculated to achieve the benefits and advantages set forth, it will beappreciated that the invention is susceptible to modification, variationand change without departing from the spirit thereof.

What is claimed is:

l. A cobalt-base alloy consisting essentially of about 18 percent toabout 26 percent chromium, about 7 percent to about 12 percent nickel,up to about 2 percent iron, about 1 percent to about 4 percentmolybdenum, about 2.5 percent to about 4.5 percent tungsten, about 2percent to about 5 percent tantalum, about 0.1 percent to about 0.4percent titanium, about 0.1 percent to about 1.0 percent zirconium,about 0.4 percent to about 0.7 percent carbon and the balance cobaltalong with conventional residual alloying elements and conventionalincidental impurities.

2. The alloy as defined in claim 1, in which the conventional residualalloying elements and conventional incidental impurities includemanganese present in an amount up to about 0.5 percent, silicon up toabout 0.5 percent, boron up to about 0.1 percent and niobium up to about2 percent.

3. The alloy as defined in claim 1, in which said chromium is present inan amount of about 23 percent to about 25 percent, said nickel ispresent in an amount of about 9 percent to about 11 percent, said ironis present up to about 1 percent, said molybdenum is present in anamount of about 1 percent to about 3 percent, tungsten is present in anamount of about 3.2 percent to about 3.8 percent, tantalum is present inan amount of about 3 percent to about 4 percent, titanium is present inan amount of about 0.2 percent to about 0.3 percent, zirconium ispresent in an amount of about 0.3 percent to about 0.6 percent, carbonis present in an amount of about 0.55 percent to about 0.65 percent andwith the balance consisting essentially of cobalt.

4. A cobalt-base alloy consisting essentially of about 18 percent toabout 26 percent chromium, about 7 percent to about 12 percent iron, upto about 2 percent nickel, about 1 percent to about 4 percentmolybdenum, about 2.5 percent to about 4.5 percent tungsten, about 2percent to about 5 percent tantalum, about 0.1 percent to about 0.4percent titanium, about 0.1 percent to about 1.0 percent zirconium,about 0.4 percent to about 0.7 percent carbon and with the balancecobalt along with conventional residual alloying elements andconventional incidental impurities.

5. The alloy as defined in claim 4, in which said conventional residualalloying elements and said conventional incidental impurities includemanganese up to about 1 percent, silicon up to about 1 percent, boron upto about 0.1 percent, and niobium up to about 2 percent.

6. The alloy as defined in claim 4, containing about 23 percent to about25 percent chromium, about 9 percent to about 11 percent iron, up toabout 1 percent nickel, about 1 percent to about 3 percent molybdenum,about 3.2 percent to about 3.8 percent tungsten, about 3 percent toabout 4 percent tantalum, about 0.2 percent to about 0.3 percenttitanium, about 0.3 per cent to about 0.6 percent zirconium, about 0.6percent carbon with the balance consisting essentially of cobalt.

1. A COBALT-BASE ALLOY CONSISTING ESSENTIALLY OF ABOUT 18 PERCENT TOABOUT 26 PERCENT CHROMIUM, ABOUT 7 PERCENT TO ABOUT 12 PERCENT NICKEL,UP TO ABOUT 2 PERCENT IRON, ABOUT 1 PERCENT TO ABOUT 4 PERCENTMOLYBDENUM, ABOUT 2.5 PERCENT TO ABOUT 4.5 PERCENT TUNGSTEN, ABOUT 2PERCENT TO ABOUT 5 PERCENT TANTALUM, ABOUT 0.1 PERCENT TO ABOUT 0.4PERCENT TITANIUM, ABOUT 0.1 PERCENT TO ABOUT 1.0 PERCENT ZICRONIUM,ABOUT 0.4 PERCENT TO ABOUT 0.7 PERCENT CARBON AND THE BALANCE COBALTALONG WITH CONVENTIONAL RESIDUAL ALLOYING ELEMENTS AND CONVENTIONALINCIDENTAL IMPURITIES.
 2. The alloy as defined in claim 1, in which theconventional residual alloying elements and conventional incidentalimpurities include manganese present in an amount up to about 0.5percent, silicon up to about 0.5 percent, boron up to about 0.1 percentand niobium up to about 2 percent.
 3. The alloy as defined in claim 1,in which said chromium is present in an amount of about 23 percent toabout 25 percent, said nickel is present in an amount of about 9 percentto about 11 percent, said iron is present up to about 1 percent, saidmolybdenum is present in an amount of about 1 percent to about 3percent, tungsten is present in an amount of about 3.2 percent to about3.8 percent, tantalum is present in an amount of about 3 percent toabout 4 percent, titanium is present in an amount of about 0.2 percentto about 0.3 percent, zirconium is present in an amount of about 0.3percent to about 0.6 percent, carbon is present in an amount of about0.55 percent to about 0.65 percent and with the balance consistingessentially of cobalt.
 4. A cobalt-base alloy consisting essentially ofabout 18 percent to about 26 percent chromium, about 7 percent to about12 percent iron, up to about 2 percent nickel, about 1 percent to about4 percent molybdenum, about 2.5 percent to about 4.5 percent tungsten,about 2 percent to about 5 percent tantalum, about 0.1 percent to about0.4 percent titanium, about 0.1 percent to about 1.0 percent zirconium,about 0.4 percent to about 0.7 percent carbon and with the balancecobalt along with conventional residual alloying elements andconventional incidental impurities.
 5. The alloy as defined in claim 4,in which said conventional residual alloying elements and saidconventional incidental impurities include manganese up to about 1percent, silicon up to about 1 percent, boron up to about 0.1 percent,and niobium up to about 2 percent.
 6. The alloy as defined in claim 4,containing about 23 percent to about 25 percent chromium, about 9percent to about 11 percent iron, up to about 1 percent nickel, about 1percent to about 3 percent molybdenum, about 3.2 percent to about 3.8percent tungsten, about 3 percent to about 4 percent tantalum, about 0.2percent to about 0.3 percent titanium, about 0.3 percent to about 0.6percent zirconium, about 0.6 percent carbon with the balance consistingessentially of cobalt.